Thermal stability of cellulose insulation in electrical power transformers – A review

Jusner, Paul; Schwaiger, Elisabeth; Potthast, Antje; Rosenau, Thomas

doi:10.1016/j.carbpol.2020.117196

Cited by 54 publications

(33 citation statements)

References 52 publications

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“…3a, 4a, and 5a clearly support hornification and coalescence of cellulose fibers for WP samples under the given elevated temperatures. This irreversible aggregation of cellulose fibrils is facilitated by high temperatures and acidic environments (Po ¨nni et al 2012) which is to be expected in thermally aged systems of oil immersed paper (Jusner et al 2021). There is no exact definition of ''hornification'' and its different aspects are still under debate, but independently of whether hornification actually is to be defined as the formation of intra-fiber links (ester or hemiacetal/hemiketal) upon thermal stress (Fernandes Diniz et al 2004) or the irreversible formation of intra-fiber hydrogen-bond networks (Kato and Cameron 1999), it most certainly results in reduced swelling of cellulose fibrils by water (i.e.…”

Section: Resultsmentioning

confidence: 98%

Analyzing the effects of thermal stress on insulator papers by solid-state 13C NMR spectroscopy

et al. 2021

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Million tons of cellulosic paper have been used for insulating coils in oil-filled electrical power transformers, thereby assuring the electricity supply for our societies. The high working temperatures in transformers constantly degrade paper insulators throughout their service life of up to 40 years. We approached the structural changes in oil-immersed cellulosic paper samples upon thermal stress in a study that compared unbleached softwood Kraft paper used as insulator paper with pure cotton cellulose paper. The model experiments used a thermal treatment in transformer oil at 170 °C for up to 14 days. The samples were characterized by means of 13C CP/MAS NMR spectroscopy, mainly based on deconvolution of the C4 resonance. An automated, fast, and reproducible C4 resonance deconvolution employing the “Peak Analyzer” tool of OriginPro 2020 (OriginLab Corporation, USA) was developed and used to exploit 13C CP/MAS NMR spectroscopy for the characterization of thermally stressed paper samples. Our results show that thermally induced structural changes depend heavily on the composition of paper, that hornification and coalescence of fibrils take place, and that the allomorph composition of cellulose crystallites is altered under the given conditions. Graphical abstract

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Section: Resultsmentioning

confidence: 98%

Analyzing the effects of thermal stress on insulator papers by solid-state 13C NMR spectroscopy

et al. 2021

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“…The temperature rise in power transformers may result from electrical faults, overloads, etc. [1,2].…”

Section: Introductionmentioning

confidence: 99%

Improving Thermal Stability and Hydrophobicity of Rutile-TiO2 Nanoparticles for Oil-Impregnated Paper Application

et al. 2021

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Thermal stress and moisture absorption can cause a synergetic negative impact on kraft paper. Among various approaches for improving the dielectric properties of kraft paper, nanotechnology has had promising results. However, the hydrophilicity of most metal oxide nanoparticles renders nanomodified kraft paper more vulnerable to thermal stress and moisture, thereby inducing degradation. In nanomodified kraft paper research, the use of TiO2 nanoparticles has yielded the most promising results. The major shortfall, however, is the hydrophilicity of TiO2. This work investigated surface modifications of rutile-TiO2 nanoparticles (NPs) for improved hydrophobicity and thermal stability. Rutile-TiO2 NPs is a nontoxic metal oxide that can withstand high temperature and is stable in chemical reactions. Two cases of surfactants were used—alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA). The intention was to increase heat resistance and reduce the surface free energy of the rutile-TiO2 NPs. The impacts of the surface modifiers on the rutile-TiO2 NPs were characterised using FT-IR, muffle furnace, analytical weight balance, and TGA. It was discovered that new functional groups were formed on the modified NPs examined through FT-IR spectra. This indicates new chemical bonds, introduced through the surface modification. The unmodified rutile-TiO2 NPs absorbed moisture, increasing their mass by 3.88%, compared with the modified nanoparticles, which released moisture instead. TGA analysis revealed that AKD- and ASA-modified rutile-TiO2 needed higher temperatures than the unmodified rutile-TiO2 to markedly decompose. AKD, however, gave better performance than ASA in that regard. As an example, those modified with 5% AKD sustained a 45% higher temperature than the pure TiO2 nanoparticles. Furthermore, in both cases of the surfactants, the higher the percent of surfactant content was, the more thermally stable the nanoparticles became. This work demonstrates the possibility of fabricating rutile-TiO2 NPs to give improved hydrophobicity and thermal stability for possible dielectric applications such as in kraft paper for power transformer insulation.

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“…Nowadays, oil-immersed transformers, whose insulation system is composed of an insulating uid (mineral oil, natural or synthetic ester) and a conductor insulation (Kraft, Crépe, DDP, Nomex®…) are widely used worldwide (Chen et al 2020; Garelli et al 2021). Cellulose insulants are the main solid insulator for the winding conductors, inter-winding, intercore and winding-to-earth insulation (Levchik et al 1998; Rao and Jarial 2019) due to its excellent insulation performance and mechanical properties at elevated temperatures (Medina et al 2017;Jusner et al 2021).…”

Section: Introductionmentioning

confidence: 99%

Fracture Toughness As An Alternative Approach To Quantify The Ageing of Insulation Paper In Oil

Fernández-Diego

Carrascal

Ortiz

et al. 2021

Preprint

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Oil-immersed transformers use paper and oil as insulation system which degrades slowly during the operation of these machines. The fast-developing electric power industry demands superior performance of electrical insulation materials which has led to the development of new materials whose measurement of the degree of polymerization has found some practical difficulties. Moreover, the increasing interest in synthetic dielectric materials replacing cellulose materials requires the use of alternative methods to the degree of polymerization to quantify the degradation of insulation solids over time. In this sense, this paper proposes the possibility of analyzing paper degradation through fracture toughness. An accelerated thermal ageing of Kraft paper in mineral oil was carried out at 130ºC during different periods of time, to obtain information on the kinetics of the ageing degradation of the paper. Double-edged notched specimens were tested in tension to study their fracture toughness. The evolution of the load-displacement curves obtained for different ageing times at the ageing temperature of 130°C was utilized to the determination of the stress intensity factor. Furthermore, different kinetic models based on this stress intensity factor were applied to relate its evolution over time as a function of the temperature. Finally, the correlation between the DP and stress intensity factor, which depends on the fiber angle, was also defined.

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Thermal stability of cellulose insulation in electrical power transformers – A review

Cited by 54 publications

References 52 publications

Analyzing the effects of thermal stress on insulator papers by solid-state 13C NMR spectroscopy

Analyzing the effects of thermal stress on insulator papers by solid-state 13C NMR spectroscopy

Improving Thermal Stability and Hydrophobicity of Rutile-TiO2 Nanoparticles for Oil-Impregnated Paper Application

Fracture Toughness As An Alternative Approach To Quantify The Ageing of Insulation Paper In Oil

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